Programed robots, master-slave manipulators, telemanipulators, and material handling devices in general have a need to measure force in articulated joints so that force and position control may be accurately exercised. For gripping devices, maximum gripping forces may be varied according to the nature of the object being grasped. In assembly operations involving a wide range of variables, adaptive control is dependent upon force and position feedback information. Where unplanned contact with an object is a risk, force feedback can be used to initiate corrective action and to prevent damage to the manipulator and/or to the object being grasped.
Particularly where there are size constraints, various indirect means are now used to measure torque in an articulated joint, such as: resilient elements in motor drive shafts; measurement of actuator motor currents and other motor parameters; pressure sensors in drive mechanisms; and strain gages in structural members.
One example of a resilient element in the drive shaft of a rotary actuator is shown in U.S. Pat. No. 4,600,357 to Coules. This patent discloses a robot end effector with two opposing pivoted jaws. A helical gear assembly connects these jaws to a single rotary drive motor with a resilient element incorporated into the drive shaft.
Rotary position sensors are incorporated into the drive shaft before and after the resilient element. The difference between the rotary position sensors provides an indication of the torque being transmitted. The position sensor located after the resilient element also provides an indication of the position of the grasping jaws.
U.S. Pat. No. 4,727,996 to Fenn et al. discloses a gripping mechanism where the actuator motor current is sensed as an indirect indication of the gripping forces. When an electric motor is used, a braking mechanism is required to hold a particular position. Only a dynamic reading is provided. As changing ambient conditions can effect the torque force, critical information is not available when the motor is in a locked position.
An electric stepper motor may be used to provide a holding force without requiring additional braking means. U.S. Pat. No. 4,535,405 to Hill et al. discloses a means to measure torque using the difference between actual motor shaft position and theoretical shaft position based upon the desired position of the joint being controlled. Actual motor shaft position is determined by a rotary position sensor at the motor. The theoretical motor shaft position is determined by a rotary position sensor at the joint axis. The reading of theoretical motor shaft position is correlated to a motor shaft position under a no load condition. Angular deviations of the motor shaft and the maximum force are limited. As motor shaft angular deviations increase, they reach a point where the stepper motor slips to the next rotor hold position.
U.S. Pat. No. 4,666,361 to Kitabatake et al. discloses the use of pressure sensors to indirectly measure torque. A rotary motor is used to drive a worm wheel assembly which is connected to an arm joint by means of a drive belt. Pressure sensor assemblies are located at opposite ends of the worm gear. An indication of the torque at the arm joint is provided by measuring the axial force present in the worm gear. The direction of the torque in the arm pivot is indicated by the active pressure sensor.
Strain gages for indirect torque measurement are used in advanced hand-like grasping devices with multiple and closely spaced joints. The Standford/JPL hand-like robot end effector has three fingers, each having three joints. (International Encyclopedia of Robots. R. Dorf, Editor. 1988, page 627). Drive motor actuating forces are transmitted by cables in flexible conduits from the drive motors located beyond the end effector wrist. An indication of torque at each joint is provided by a means to measure tension in the cables by strain gages incorporated in the cable guiding structure. The drive motors have rotary position sensors on their shafts to provide an indication of the angular position of the joint.
The Utah/MIT hand-like robot end effector has four fingers, including an opposing thumb. (International Encyclopedia of Robots, R. Dorf Editor, 1988, page 627-628). In this design, each finger has four joints. Each joint is actuated by a pair of tendons connected to a pair of pneumatic cylinders located beyond the end effector wrist. Strain gages are incorporated into the guiding structure routing each tendon to provide an indication of the torque at each joint. Position sensing is provided directly at each joint by a Hall effect sensor.
In all of the above examples, the sensors which do not read directly at the joint axis are subject to errors introduced by friction and tolerance variations among interrelated components.
The present invention provides a novel way to provide angular position and torque information in an articulated joint wherein both sensors are integrated into the joint, improving accuracy and providing a more compact assembly. The mechanism is simplified, providing cost and reliability advantages which are not made obvious by the known prior art.